The present invention relates to injection molding machines in general and, in particular, to a new and useful injecting molding machine having controls which permit smooth and pressure surge-free transition between a speed controlled ejection phase, a dwell pressure phase and a final solidification phase.
Injection molding machines are known in which the actual value of the pressure in the hydraulic cylinder is picked up by a sensor and compared with a settable desired value pattern, and the pressure in the hydraulic cylinder is controlled by means of a hydraulic actuator to make the actual and desired values coincide. Such a control device has the disadvantage that a varying viscosity of the plastic material to be injected results in varying pressures in the tool and, consequently, in irregularities in the quality of the molded piece.
Injection molding machines are also known in which the actual value of the pressure in the tool is picked up by a sensor and compared with a settable pattern of desired values, and the desired and actual values are forced to coincide by means of a hydraulic actuator. Such a control device has the disadvantage that, at the start of the dwell pressure phase, a pressure surge-free transition from the injection phase to the dwell pressure phase is not ensured. That is, in the prior art control device, the machine setter must preset a threshold value of the pressure at which the switching to the dwell pressure phase is to be effected. If this threshold value is too high or too low, pressure peaks or drops based on dynamic control actions are produced in the tool which may damage the molded piece. Another disadvantage of this prior art control device is that, with an extended dwell pressure period, the pressure transmission from the hydraulic cylinder to the sensor in the tool drops to zero, so that at the end of the dwell pressure time, the function of the control device is no longer satisfactory. The hydraulic pressure is controlled to either reach its maximum or to reach zero, which may load the tool or relieve it from load and, thereby, seriously damage the molded part.
In another prior art injection molding machine, the pressure is also picked up in the tool, compared with a settable pattern of the desired values and controlled by a corresponding hydraulic actuation to coincide therewith. To prevent the tool from being exposed to excessive or insufficient pressure in the final portion of the dwell pressure period, a switching to a steadily or linearly dropping hydraulic pressure is provided at a predetermined instant within the dwell pressure phase. This device is an improvement over the design described above. However, it has the disadvantage that pressure surges may occur during the switching, which again results in damage to the molded piece.